Electrostatic desalting of crude oil has been practiced for many years. Desalting is necessary prior to further processing to remove salts and to prevent deposition of materials suspended in the oil in lines and processing equipment. While this was a relatively troublefree procedure in the past, several factors have necessitated the development of improved processes.
The character of raw crude oil from many sources is changing because of lowering levels of oil fields which results in the delivery of heavier crudes. Chemicals added during oil recovery further add to the problem.
More stringent regulatory requirements have increased the attractiveness of improved procedures. Water quality is monitored by regulatory authorities and all materials, except oil, discharged from desalting processes are listed waste.
In the typical desalter process, crude oil is mixed with water, demulsifiers and anionic materials. The mix is passed into a desalter assembly where it is exposed to an applied electrical field causing an induced dipole to be formed in each water droplet which causes electrostatic coalescence of the water droplets which have suspended particles including metals such as iron. The metals to some extent combine with the anionic additives and form aqueous insoluble salts and these, together with other agglomerated particulates, fall into the heavier aqueous phase. The streams of desalted crude oil and aqueous waste are separately discharged from the desalter.
A persistent problem, however, results from the presence of a stable emulsion which forms at the interface of the oil and the aqueous phase in the desalter. This rag or cuff layer collects drilling mud, water insoluble salts of many metals particularly iron, production chemicals, silt and the like. When the cuff layer becomes enlarged, capacity of the desalting apparatus is reduced, while operating problems such as arcing across the electric grid may result.
Many desalting modifications have been introduced in efforts to break the interface emulsion and obtain clean splits between the oil and the aqueous phase. U.S. Pat. No. 4,722,781 teaches recycling of a major portion of the interface into incoming crude oil to break the emulsion, with a small portion of the emulsion being separated and diluted with several volumes of light hydrocarbon such as kerosene or naphtha. This approach presents several problems. The bulk of the solids in the interface are added back into the incoming crude oil to increase the level of contamination; and the diluent, being at least 3 to 10 times the volume of cuff material, requires increased equipment capacity.
U.S. Pat. No. 4,116,790 addresses the problem of process limitations experienced in separating oil-water mixtures. This process consists of applying an electric field to coalesce small liquid drops into much larger drops, followed by centrifugation of the entire mixture to speed separation. No suggestion of what magnitude of "g" forces needed are described, the disclosure stating only that the forces be great enough to speed separation but not so great as to refragment the droplets. This process is not concerned with the interface emulsion formed after application of the electric field, and which persists after separation of the coalesced material from the oil phase, as the entire mix is centrifuged. Centrifuging the entire crude oil stream is highly inefficient and costly an separation of any emulsion present is poor.
U.S. Pat. No. 3,923,643, concerned with reprocessing of used lubricating oil, induces agglomeration of suspended solids by flashing off water and light hydrocarbons followed by holding the treated heavy oil at a temperature of 500.degree. to 700.degree. F. for 1 to 12 hours. This method is directed only to removal of heat-agglomerated solids and not to the interface formed during desalting of virgin crude oil which is resistant to separation.
Another process for treating used lubricating oils is taught by U.S. Pat. No. 4,250,021. This utilizes addition of surfactant to oxalate, chromate, phosphate and sulfate anions to form metal insoluble salts while inhibiting formation of an emulsion. After treatment the entire batch of treated oil is passed through a centrifuge.
While these prior art methods are useful in limited applications they do not satisfy the present need for a more efficient process.